What is the NLA Editor used for?
Short answer: High-quality character animation is hard work even when it is done by experts. A lot of work goes into every second. Nonlinear animation is an attempt to speed things up, but note that it could sacrifice some quality. 

Longer answer: When you start animating, you can be successful using only keyframes, but after a while you find yourself animating similar motions again and again, and you want to reuse your work, just like you reuse node groups. You also want to be able to mix and blend overlapping motions in a nondestructive way. You might want to create an animation layer for the broad movements and one for the refinements. Or a layer of face motions and one for the body, and combine them according to the story. Nonlinear animation is used for achieving these goals.

Note that the quality of automatically reused and blended motions might not be as good as the quality of a hand-crafted keyframe animation, but they could be fine for background characters. It could be a tradeoff between quality and time.

The following video explains the "how" part with an example: https://www.youtube.com/watch?v=W4TkA5P5Qes